In recent years, as electronic products have had lighter weights and smaller sizes, there has been an increasing demand for various printed-circuit boards. Among the printed-circuit boards, flexible printed-circuit boards (FPCs) have been increasingly demanded in particular. A flexible printed-circuit board is produced from a flexible laminate obtained by laminating metal foil such as copper foil on an insulating film.
Such a flexible laminate is manufactured by a method for laminating metal foil onto a surface of a flexible insulating film by heating and pressure bonding via an adhesive. Preferred examples of the flexible insulating film include a polyimide film. Commonly-used examples of the adhesive include thermosetting adhesives such as epoxy adhesives and acrylic adhesives. Such a flexible printed-circuit board manufactured with use of a thermosetting adhesive is called “three-layer FPC” because of its three-layer structure of a substrate, an adhesive, and metal foil.
It should be noted that a polyimide film is usually manufactured by a flow-casting film-forming method by which: a polyamic acid solution serving as a precursor is extruded with use of an extrusion die onto a casting belt in the form of a film; a self-supporting film is obtained by evaporating a solvent; and the self-supporting film is dried by heating.
A thermosetting adhesive for use in a three-layer FPC offers an advantage of enabling adhesion at a relatively low temperature. However, a three-layer FPC manufactured with use of a thermosetting adhesive is limited in properties such as heat resistance, bendability, and electric reliability. Proposed in view of this is an FPC obtained by forming a metal layer directly on a polyimide film without using a thermosetting adhesive. Such an FPC excels in heat resistance, bendability, electric reliability, and other properties. Such an FPC is called “two-layer FPC” because it is obtained by forming a metal layer directly on a polyimide film.
Known examples of a method for manufacturing a laminate for use in a two-layer FPC include (a) a casting method for imidizing polyamic acid, serving as a polyimide precursor, which has been flow-cast and applied onto metal foil; (b) a metalizing method for forming a metal layer directly on a polyimide film by sputtering and plating; and (c) a laminating method for laminating a polyimide film and metal foil with a thermoplastic polyimide sandwiched therebetween. Among these methods, the laminating method can deal with a wider range of thickness of metal foil than the casting method, and is lower in apparatus cost than the metalizing method.
Usually, the laminating method employs an adhesive polyimide film obtained by forming a thermoplastic polyimide layer(s) on one surface or both surfaces of a highly heat-resistant polyimide film.
There are various methods devised as a method for manufacturing such an adhesive polyimide film. Highly productive among these methods is an extrusion casting method by which after a highly heat-resistant polyimide resin solution or a precursor solution thereof and a thermoplastic polyimide solution or a precursor solution thereof are extruded with use of a multilayer coextrusion die onto a casting belt so as to form a multilayer liquid film, the multilayer liquid film is dried by heating.
It is necessary for a polyimide film to be entirely uniform in thickness. Furthermore, it is necessary for a multilayer polyimide film not only to be uniform in total thickness, but also to be uniform in thickness of each layer. In a multilayer polyimide film that is uniform in total thickness but is not uniform in thickness of each layer, the layers have different coefficients of linear expansion; therefore, the multilayer polyimide film suffers from the nonuniformity of coefficient of linear expansion across the whole film.
Further, a polyimide film that is not uniform in total thickness or thickness of each layer has a partially noncontact portion or a dimensionally changeable portion undesirably generated when metal foil such as copper foil is laminated on the polyimide film.
The extrusion casting method makes it easy to obtain a film uniform in total thickness or thickness of each layer in the length direction (MD direction). However, the extrusion casting method does not make it easy to obtain a film uniform in total thickness or thickness of each layer in the width direction (TD direction).
Each of Patent Documents 1 and 2 discloses a method for, in molding a thermoplastic resin by extrusion, adjusting the thickness of a sheet or a film to be formed. Patent Document 1 discloses a method for adjusting the space between lips of an extrusion die by attaching, to the lips, heat bolts, i.e., bolts that expand with heat. Patent Document 2 discloses a method for adjusting the thickness of a sheet or a film by adjusting the discharge rate of a molten resin by changing the temperature of lips of an extrusion die by attaching lip heaters, i.e., heating means to the vicinity of the lips.
Patent Document 3 discloses a method for, in order to adjust the thickness of a sheet or a film in the width direction, attaching a plurality of heat bolts or lip heaters to lips so that the heat bolts or lip heaters are placed in the width direction (usually at intervals of 7 mm to 40 mm). The method makes it possible to directly control a thickness distribution. However, the method suffers from the disadvantage of being complex in apparatus and highly complex in control program. Furthermore, in the case of manufacture of an adhesive polyimide film with use of a multilayer coextrusion die for the purpose of manufacturing a multilayer film, the method makes it possible to control a distribution of total film thickness, but impossible to control a distribution of thickness of each layer.
Patent Document 4 discloses a technique for, in a feed-block multilayer coextrusion die, uniforming a distribution of film thickness in the width direction by uniforming the flow rate of each molten resin by heating each molten resin separately in a flow channel provided in the feed block. However, the method makes it necessary to adjust the temperatures of resins in a large number of flow channels provided in a narrow space inside of the feed block. Furthermore, the method of Patent Document 4 makes it difficult to measure viscosity, and therefore makes it also difficult to perform feedback control.
As described above, there is a known method for controlling the thickness of a multilayer thermoplastic resin film. However, there is no known method for controlling the thickness of a multilayer polyimide film manufactured with use of a solution. It is difficult to apply, to the manufacture of a multilayer polyimide film, the method for controlling the thickness of a multilayer thermoplastic resin film. Further, there is no known multilayer polyimide film, manufactured with use of a multilayer coextrusion die, which is uniform in thickness of each layer.    Patent Document 1: U.S. Pat. No. 3,940,221 Specification    Patent Document 2: Japanese Examined Patent Application Publication No. 59050/1982 (Tokukosho 57-59050)    Patent Document 3: Japanese Unexamined Patent Application Publication No. 206615/1985 (Tokukaisho 60-206615)    Patent Document 4: Japanese Unexamined Patent Application Publication No. 309770/1999 (Tokukaihei 11-309770)